Astraeus flies Gatwick to Malabo and Bata on a 737-700. Flight time is just over 7 hours.

They also operate the 737-700 transatlantic on scheduled service between Gatwick and St.John's, NL. Of course, this is barely 5 hours (the record is 4h32m) so not really long, but still comment worthy.

Kenya Airways flies the 737-700 between Nairobi and Bamako which can push 7.5 hours with bad winds.

Egyptair also flies between Cairo and Accra on a 737-800 for just under 7 hours flight time.